Ugly fix to allow overriding option `ssh_to` for a cluster which is already started
Squashed 'elasticluster/providers/ansible-playbooks/' changes from 15493d4..f4141ea

f4141ea cosmetic changes
34137b7 Remove `blacs-openmpi` package which is not available on CentOS 6 (apparently)
37770c9 Add support for SLURM on CentOS 6 (Needs to download and compile slurm)
98af8db update `nfsexport` module: add option `state` to ensure the filesystem is mounted _and_ exported
a002181 Fix typo
4a3feb9 If hadoop_tar_url variable is defined, an installation will be performed using a tar file downloaded from {{hadoop_tar_url}} instead of using the deb files defined in `deburl` or `rpmurl`.

git-subtree-dir: elasticluster/providers/ansible-playbooks
git-subtree-split: f4141ea705461e1eb2ab6fc3cbe037e59f309ab7
Update ansible playbooks